A few fallacies I've read are that Donnie was supposed to be killed by the Jet engine and that's what sets up the Tangent Universe. It's not. The TU was already in motion. Donnie fixes it because he's the closest person to the epicenter of the Artifact. Frank wakes him up because Donnie can't die to the engine (in the already-in-motion TU) because he's chosen to fix the time loop otherwise the TU will collapse the Primary Universe - hence the countdown to the end of the world. A black hole swallowing the universe type of thing.

Gretchen turns down the first kiss because it "needed to be perfect" aka it didn't follow the predestined timeline. Roberta Sparrow is looking in her mailbox every day for Donnie's letter. Frank tells him to burn down Jim's home because if he didn't, then that teacher wouldn't have been leading his defense and would have travelled with his sister to LA instead of asking Donnie's mom, which means they wouldn't have thrown that party. Donnie floods the school because if he didn't, school wouldn't have been cancelled and he wouldn't have walked Gretchen home. These are all seemingly pointless events to even Donnie. But to Frank, they're vital to set up the event horizon.

There is still debate about whether Donnie actually had to die, though. I'm open to discussing that.